1. Field of the Invention
The invention relates to a system for providing hearing assistance to a user, comprising at least one audio signal transmission unit comprising an audio signal source, typically a microphone arrangement, and means for transmitting audio signals from the audio signal source via a wireless radio frequency link to a left ear receiver unit worn at the user's left ear and a right ear receiver unit worn at the user's right ear. Typically, each of the receiver units is connected to a hearing aid, so that the user's hearing can be stimulated according to the audio signals of the audio signal source.
2. Description of Related Art
Typically, such wireless microphones are used by teachers teaching hearing impaired persons in a classroom (wherein the audio signals captured by the wireless microphone of the teacher are transmitted to a plurality of receiver units worn by the hearing impaired persons listening to the teacher) or in cases where several persons are speaking to a hearing impaired person (for example, in a professional meeting, wherein each speaker is provided with a wireless microphone and with the receiver units of the hearing impaired person receiving audio signals from all wireless microphones). Another example is audio tour guiding, wherein the guide uses a wireless microphone.
Typically, the wireless audio link is an FM (frequency modulation) radio link operating in the 200 MHz frequency band. Examples for analog wireless FM systems, particularly suited for school applications, are described in EP 1 864 320 A1 corresponds to WO 2006/104634 A2 and WO 2008/138365 A1.
In recent systems the analog FM transmission technology is replaced by employing digital modulation techniques for audio signal transmission, most of them working on other frequency bands than the former 200 MHz band.
U.S. Pat. No. 8,019,386 B2 relates to a hearing assistance system comprise a plurality of wireless microphones worn by different speakers and a receiver unit worn at a loop around a listener's neck, with the sound being generated by a headphone connected to the receiver unit, wherein the audio signals are transmitted from the microphones to the receiver unit by using a spread spectrum digital signals. The receiver unit controls the transmission of data, and it also controls the pre-amplification gain level applied in each transmission unit by sending respective control signals via the wireless link. Mixing of the received audio signals is controlled such that the signal with the highest audio power is amplified with unity gain, and the other signals are attenuated by 6 dB.
International Patent Application Publication WO 2008/098590 A1 relates to a hearing assistance system comprising a transmission unit having at least two spaced apart microphones, wherein a separate audio signal channel is dedicated to each microphone, and wherein at least one of the two receiver units worn by the user at the two ears is able to receive both channels and to perform audio signal processing at ear level, such as acoustic beam forming, by taking into account both channels.
International Patent Application Publication WO 2011/098142 A1 relates to a hearing assistance system comprising a plurality of wireless microphones, a relay unit and a left ear receiver unit and a right ear receiver unit, wherein the relay unit as adapted to mix the audio signals of the different transmission units and to transmit the mixed audio signal in a manner that a different audio signal is received by the right ear receiver unit and by the left ear receiver unit in order to enable spatial hearing by the user of the receiver units.
European Patent Application EP 2 099 236 A1 relates to a hearing aid fitting method using simulated surround sound, wherein different head related transfer functions are applied to test audio signals supplied to the hearing aid.
U.S. Pat. No. 8,369,551 B2 relates to a hearing aid receiving audio signals via a wireless audio link, wherein the distance to the audio signal transmitter is monitored by monitoring the reception quality.
European Patent Application EP 1 303 166 A2 relates to a hearing aid which is capable of determining the angular position of a speaking person.
International Patent Application Publication WO 2009/072040 A1 relates to a right ear hearing aid and a left ear hearing aid which are capable of localizing a sound source for controlling acoustic beam forming in each of the hearing aids.
U.S. Patent Application Publication 2007/0230714 A1 relates to a binaural system comprising a right ear hearing aid and a left ear hearing aid, which are capable of exchanging audio signals via a wireless link, wherein a delayed sound signal is transmitted from one of the hearing aids to the other one in order to achieve a time delay between the sound provided by the right hearing aid and the sound provided by the left ear hearing aid; this delay mimics how the ears would naturally hear a sound coming from one side from the head.
International Patent Application Publication WO 2009/056922 A1 relates to a telephone system, wherein the voices of different participants of a telephone conference are supplied as a mixed stereo signal to two ears of a listener in order to create a spatial perception of the different voices, thereby supporting the listener in distinguishing the different persons.
Various methods are known for estimating the angular localization of a source of a radio frequency (RF) signal with regard to a RF receiver. International Patent Application Publication WO 2009/147662 A1 relates to a method for determining whether a target is within a direction sector of interest of a direction finder, wherein the direction finder comprises two antennas arranged in a broad-side configuration. U.S. Pat. No. 6,748,324 B2 relates to a method of estimating the angular localization of a wireless device by a direction of arrival (DOA) measurement. European Patent Application EP 2 000 816 A2 relates to a communication system comprising a mobile phone in a LAN, wherein the angle of arrival of a RF signal and a receiver device is estimated, wherein the transmitting device includes two directional antennas which are tilt relative to each other and with regard to the front of the transmitting device, and wherein the receiving device includes a directional antenna having directivity toward the front of the receiving device. International Patent Application Publication WO 2008/112765 A1 relates to a car finder, wherein the car is provided with a RF signal source and wherein the direction finding device is provided with a directional receiver antenna, and wherein the omni-directional field created by the RF signal transmitter is analyzed by a direction sweep of the receiver antenna, with the RSSI (received signal strength indication) being measured during the sweep.
U.S. Pat. No. 5,905,464 relates to a binaural system comprising two ear phones and an RF antenna having a single analysis axis which is parallel to a line connecting the two ears, which system is used for estimating the angular localization of a source of an RF signal representing a spatial mark and which generates an audio signal representative of the angular direction of the RF signal source; the audio signal may be distributed on to the two ear phones in such a manner that a spatial hearing impression is created which indicates the direction of the RF signal source. The system may be used, for example, by persons working in a dangerous, low-visibility zone, such as firemen.